1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method performed in the image forming apparatus, and a program used in the image forming apparatus. More particularly, the present invention relates to an image forming apparatus that can effectively conduct color misregistration adjustment for forming a high quality image by sequentially overlaying respective single color images formed on a plurality of image bearing members to directly or indirectly transfer onto a recording medium, an image forming method used for the image forming apparatus, and a program used to perform the image forming method in the image forming apparatus.
2. Discussion of the Related Art
Related art image forming apparatuses for forming a color image employ a tandem type configuration in which respective single color toner images are carried on a plurality of image bearing members and transferred onto a transfer member such as a recording medium or an intermediate transfer member so as to form a full color image.
Such color image forming apparatuses may cause a color misregistration or color misregistration problem. Specifically, when at least one image bearing member of a plurality of image bearing members transfers the single color toner image carried thereon onto a position relatively off its correct position, a color misregistration on a full color image may occur.
Such color misregistration can be caused by various positional deviations. The positional deviations include a vertical scanning misregistration or scanning misregistration in a sub-scanning direction of each toner image and a misregistration due to skew.
FIGS. 1 and 2 are views for explaining color misregistration.
FIG. 1 shows a vertical (sub-scanning direction) scanning color misregistration, which is a deviation on a transfer member in the vertical scanning direction or in a transfer member travel direction from an ideal image formation line (position at which toner image is to be properly formed) indicated by a solid line in FIG. 1.
FIG. 2 shows an inclination of an image on the transfer member in the vertical scanning direction or in the transfer member travel direction from an ideal image line indicated by a solid line that extends in a direction perpendicular to the transfer member travel direction in FIG. 2.
The above-described positional deviations can be results of a positional misregistration of replaced components or parts of an image forming apparatus. Actually, the positional deviation most frequently occurs due to expansion and contraction of image writing parts, such as reflection mirrors, according to temperature change.
The misregistration in the sub-scanning direction can be reduced by adjusting an image writing timing. The misregistration due to skew can be reduced by changing the installed condition, which includes setting the values of the angle, direction, position, etc., by a drive unit. It is known that periodical adjustment of positional deviations of an image forming apparatus can contribute to a reduction in frequency of color misregistration due to expansion and contraction of image writing components or parts caused by the temperature change.
That is, after respective single color toner images formed on the plurality of image bearing members have been transferred onto a transfer member, optical sensors provided in the image forming apparatus may read the respective toner images and detect relative positional deviation to each other. Based on the detection results, the image forming apparatus may adjust the image writing timing and/or the installed condition of the image writing components or parts.
One technique has been proposed for use in an image forming apparatus, in which a timing of positional deviation adjustment is determined based on the detection result obtained by a temperature sensor (or temperature sensors) disposed in the image forming apparatus.
By employing this technique, the color misregistration can be reduced by conducting the next positional deviation adjustment at the point in which the temperature inside the image forming apparatus has changed and reached a predetermined amount from a point that the positional deviation adjustment is performed. In other words, in a case in which the next positional deviation adjustment is conducted when the temperature has reached the point in which a target image writing component or part has expanded or contracted by a predetermined degree, the color misregistration can be reduced. The above-described technique, however, has a disadvantage that a temperature sensor may increase the cost.
Different techniques have proposed to cause a related art image forming apparatus to conduct the above-described positional deviation adjustment each time after a predetermined amount of prints are output.
Such related art image forming apparatus can reduce the color misregistration without the above-described temperature sensor(s), which can avoid an increase in cost. However, even when the temperature inside the image forming apparatus does not change remarkably, the positional deviation adjustment can be conducted. This can undesirably increase the number of operations of the positional deviation adjustment.
As described above, there are the positional deviation adjustment based on the detection result obtained by the temperature sensor(s) and the positional deviation adjustment based on the predetermined amount of printouts. Consequently, both of these techniques have advantages and disadvantages. Therefore, it is preferable to employ the positional deviation adjustment based on the predetermined amount of printouts because this positional deviation adjustment can be performed at a lower cost. Further, an image forming apparatus including the positional deviation adjustment based on the predetermined amount of printouts can be mass-produced for a general user.
However, even though it is expected to mass-produce such image forming apparatuses for the general user, it is not preferable to increase the amount of color misregistration and the number of times to force users to wait, which may be referred to as “the number of waits” because of the positional deviation adjustment. Accordingly, the number of waits and the amount of color misregistration may need to be limited to respective allowable ranges.
Unfortunately, in order to surely maintain the amount of color misregistration within its allowable range when the positional deviation adjustment is performed based on the predetermined amount of printouts, the general user has to wait the number of times that is greater than the allowable number of waits.
Specifically, the inventors of the present invention have found that the expansion and contraction of the image writing components or parts may be caused due to the change of temperature inside an image forming apparatus, particularly inside an image writing device in an image forming apparatus.
The inventors of the present invention have also found that when the image forming apparatus is installed in a normal indoor environment, the temperature inside the image writing device in a standby mode does not remarkably change.
The temperature inside the image writing device may remarkably change when a large amount of a serial printing job is performed. During the serial printing job, the temperature inside the image writing device keeps increasing according to an increase of the number of prints.
According to a result of a test performed by the inventors of the present invention, the temperature inside the image writing device increased while conducting a serial printing job from the first sheet to the 1000th sheet.
In a case in which no positional deviation adjustment is conducted during the printing operation, the amount of the color misregistration keeps increasing according to an increase of the temperature inside the image writing device.
On the other hand, in a case in which a positional deviation adjustment is conducted by a predetermined amount of print sheets, the amount of color misregistration can be adjusted and reset to the initial value.
Further, it is natural that the increasing amount of color misregistration per one print sheet for a serial printing job can vary according to the size of the print sheet. Specifically, as the size of a print sheet is larger, the driving period of the image writing device per one print sheet becomes longer, and an amount of increase of the temperature inside the image writing device for one print sheet becomes greater.
Therefore, in order to maintain the amount of color misregistration within the allowable range, the number of trigger print sheets that are a start condition of the positional deviation adjustment may need to be set based on the assumption that the maximum size of various sizes of usable print sheets is generally used.
With the above-described condition, it has been found that the number of trigger print sheets needs to be set to a significantly small number to prevent the general user from waiting longer than the allowable range of the number of waits.
Alternative to the number of trigger print sheets, a count value of the driving period of the image writing device can be used. With the above-described count value, the positional deviation adjustment can be conducted at an appropriate timing with the increase amount of the temperature inside the image writing device, without increasing the number of waits.
To perform the positional deviation adjustment with the above-described condition, however, a new configuration or structure for counting the driving period of the image writing device may be additionally required, which can cause an increase in cost.